Conversion layers on metals are extensively used in industry, inter alia, to increase the resistance to corrosion, as a pretreatment for subsequent painting, to facilitate non-cutting cold working and running-in operations (reduction of sliding friction) and for electrical insulation. Layers based on the phosphates of Zn, Mn, Fe, Ca, Ni, Mg, and other cations have achieved special significance, and the layer may contain one or more of these cations.
An important parameter of the conversion layers is their weight per unit of area (also referred to herein as wt/A, specific weight, basis weight) because different ranges of that parameter will give optimum results in different fields of application and that parameter must be kept within a more or less narrow range for a trouble-free series production.
The weight per unit of area, which is also a measure of thickness for a given layer composition, is usually stated in grams of the layer material per square meter of the surface area (g/m.sup.2) and can be measured by the stripping method (dissolution method) involving differential weighing or by a determination of one or more components of the layer by chemical analysis, X-ray fluorescence, infrared absorption, GDOS (glow discharge optical spectroscopy) and/or other methods.
The stripping method is a destructive test and for this reason is less suitable for production control. X-ray fluorescence, infrared absorption and glow discharge optical spectroscopy require expensive analyzing apparatus. Probing devices that make use of the weakening of the adhesive force of a permanent magnet, or the influencing of the magnetic flux or the weakening of the induction effected by a high-frequency alternating current are less suitable, as a rule, for determining basis weight of conversion layers because the sensing heads may locally damage the layer and because the accuracy of the measurement is low in the range of the thicknesses of the conversion layers used.